It is known in the art that a stress corrosion cracking occurs in a weld of piping of a recirculation system of a boiling light-water reactor plant. Further, existing thermal power generating facilities are toward age deteriorations; in actual fact, European countries and the United States have experienced accidents in which high temperature steam pipes of aging thermal power plants ruptured due to cracking in a heat-affected zone caused by the occurrence of creep voids and the coupling thereof. From these problems, needs for non-destructive inspection to a weld of a thick pipe such as a weld of piping of a power generation plant, in particular, needs for sizing for detecting a generating end and a terminating end of a flaw and accurately measuring a flaw height have been increased.
As ultrasonic flaw detection methods capable of measuring a flaw height, there are, for example, a normal flaw detection method of normally transmitting an ultrasonic wave onto a flaw detection surface by a normal probe and receiving the ultrasonic wave by the normal probe therefrom, an angle flaw detection method of causing an ultrasonic wave to be incident on a flaw detection surface obliquely and receiving an echo of an edge (Non-Patent Document 1), a TOFD (Time of Flight Diffraction) method by which a longitudinal wave is caused to be incident from one of a pair of angle probes disposed by facing each other and a diffracted wave generated at an edge of a flaw is received by the other of the probes (Non-Patent Document 2), and further a measurement method called an SPOD (Short Path of Diffraction) method proposed by the inventors (Non-Patent Document 3). Meanwhile, in the SPOD method, an angle probe is combined with a normal probe, an ultrasonic wave pulse is caused to be obliquely incident on a flaw detection surface, a diffracted wave generated at an edge of a flaw is received by the normal probe above the flaw, and a flaw height is determined from the difference between the arriving times of as component which directly propagates above the flaw and a component which propagates above the flaw after it is reflected on the back surface of a member to be inspected.
In addition to the ultrasonic flaw detection methods described above, there are also proposed, for example, a method for using a secondary creeping wave which is generated when an ultrasonic transverse wave is reflected on the back surface of a specimen and an ultrasonic flaw detection method using a phased array probe.
Non-Patent Document 1: The Japanese Society for Non-Destructive Inspection, “Flaw Height Measuring Method by Tip Echo Techniques Standardized by The Japanese Society for Non-Destructive Inspection”, published on Jun. 1, 1997.
Non-Patent Document 2: The Japanese Society for Non-Destructive Inspection, “Flaw Height Measuring Method by TOFD Method Standardized by The Japanese Society for Non-Destructive Inspection”, published on Dec. 1, 2001.
Non-Patent Document 3: FUKUTOMI Hiroyuki, LIN Shan, OGATA Takashi “Proposal of Simple Flaw Sizing Method in Ultrasonic Flaw Detection Test”, Program & Abstracts of Second Academic Lecture, The Japan Society of Maintenology, 2005.